1. Field of the Invention
The present invention relates to an exhaust gas purifying system for an internal combustion engine.
2. Description of Related Art
In an internal combustion engine, such as a diesel engine, when lean burn combustion takes place, NOx (nitrogen oxide) is exhausted into the air. It has been proposed to provide an NOx storage and reduction (deoxidization) catalytic converter (hereinafter referred to as “NOx catalytic converter”), which includes an NOx storage and reduction (deoxidization) catalyst, in an exhaust system of a vehicle to purify NOx contained in the exhaust gas. The NOx catalytic converter absorbs NOx contained in the exhaust gas when an air/fuel ratio of the exhaust gas is in a lean range. The NOx catalytic converter deoxidizes (reduces) and removes its absorbed NOx with aid of NOx deoxidizing agent (reducing agent), such as HC and CO, when the air/fuel ratio of the exhaust gas is in a rich range.
When the amount of the absorbed NOx in the NOx catalytic converter reaches a saturation range and thereby reaches its absorbable limit, the NOx purifying performance of the NOx catalytic converter decreases. Thus, an NOx deoxidization control operation for deoxidizing and removing the absorbed NOx of the NOx catalytic converter is performed to limit the decrease of the NOx purifying performance of the NOx catalytic converter. Specifically, rich combustion is temporarily performed in the internal combustion engine, so that the deoxidizing agent, such as HC and CO, contained in the exhaust gas produced at the time of the rich combustion is supplied to the NOx catalytic converter to deoxidize and remove the absorbed NOx at the NOx catalytic converter. This technique is generally referred to as “rich purge” or “rich spike”.
When the internal combustion engine is used for a long period of time, sulfur components contained in the fuel are absorbed and accumulated by the NOx catalytic converter. This phenomenon is called as “sulfur poisoning”. The sulfur poisoning significantly decreases the purifying performance of the NOx catalytic converter. In view of this, there has been proposed a technique for determining the decrease in the purifying performance of the NOx catalytic converter at the time of executing the rich purge. For example, an oxygen concentration sensor is provided on a downstream side of the NOx catalytic converter, and the purifying performance of the NOx catalytic converter is determined based on a measurement of the oxygen concentration sensor (see, for example, Japanese Unexamined Patent Publication No. 2000-34946, which corresponds to U.S. Pat. No. 6,244,046). That is, at the time of performing the rich purge, when the deoxidization of the absorbed NOx is completed in the NOx catalytic converter, the air/fuel ratio on the downstream side of the NOx catalytic converter is shifted into the rich range, and this shift is sensed with the oxygen concentration sensor to determine the completion of the NOx deoxidization in the NOx catalytic converter. In this case, when the NOx purifying performance of the NOx catalytic converter decreases, i.e., when the absorbable amount of NOx decreases, the timing of shifting in the air/fuel ratio into the rich range is advanced. Therefore, it is possible to determine the decrease in the purifying performance, i.e., catalyst deterioration of the NOx catalytic converter based on the required time period, which is required to shift the air/fuel ratio into the rich range on the downstream side of the NOx catalytic converter.
Furthermore, besides the rich purge described above, there is also known another technique for deoxidizing and removing the absorbed NOx at the NOx catalytic converter. In this technique, unburnt fuel (HC), which serves as deoxidizing agent, is supplied to the NOx catalytic converter through a fuel adding valve that is provided to the exhaust pipe. This technique is advantageous in the case where the increase in the injection quantity of fuel into the cylinders of the internal combustion is not desirable. However, when the fuel is directly supplied from the fuel adding valve into the exhaust pipe, only the concentration of HC, which serves as the deoxidizing agent, in the NOx catalytic converter becomes excessively high. In this excessive state of HC, the result of the purifying performance determining operation, which is performed at the time of the NOx deoxidization control operation, may possibly become erroneous.